With the demand of higher speed internet access to the home, new technologies known as Digital Subscriber Line (DSL) have been invented. Communication lines or cables that work sufficiently well for standard telephone do not always work well for different types of DSL.
The communication cable can be any copper pair line for use in ISDN DSL (IDSL), Asymmetric DSL (ADSL), High bit rate DSL (HDSL), Symmetric DSL (SDSL), Single-Pair High-speed DSL (SHDSL), and (Very High Bitrate DSL (VDSL or VHDSL) communication systems, as well as all other DSL-type technologies (herein-after, “xDSL” indicating all the various DSL technologies and line codings).
A time domain reflectometer (TDR) can be used in locating transmission line faults. Time Domain reflectometers measure transmission line impedance and determine if there is any impedance discontinuity in the transmission line.
TDR testing typically involves a step voltage generator launching a fast edge through a resistor into a transmission line under investigation. Alternatively, a pulse generator may be used for launching a short pulse into the transmission line. The incident and reflected voltage waves are then monitored by a receiver, e.g. an oscilloscope, downstream from the resistor. This echoing technique reveals at a glance the characteristic impedance of the line under investigation in the time domain and shows the position and nature (e.g. high or low impedance) of each impedance discontinuity along the line under test. The location of each impedance discontinuity may be calculated from a time offset between the launch time of the pulse or a fast edge of the pulse and the arrival time of reflected pulses or pulse edges.
An impedance discontinuity often corresponds to a fault in the communication cable. The types of faults that a TDR can detect are most commonly associated with anomalies such as open circuits (opens), short circuits (shorts), bridged-taps and wet sections.
As the transmission lines under investigation become longer, electrical signal loss in the line increases so that it becomes increasingly harder to detect such anomalies. In most cases it takes considerable training and practice for a telephone technician to discriminate between the various anomalies. In order to locate them, even the most experienced telephone technician must often take measurements at several locations, sometimes disconnecting sections of line.
Typically, in standard TDR prior art practice, a “launch pulse” is sent out into a line which is part of a network of cables. Receivers are used to monitor the line for any return signals, which are generally displayed as a TDR trace on a screen with a time-base on the x-axis and signal amplitude on the y-axis. Thus, the arrival time and amplitude of return pulses, which are reflections from discontinuities in the line impedance at various locations in the line, can be measured and analyzed. Given a pulse propagation velocity in the line, the arrival time of a return pulse depends on the location of the corresponding anomaly, while the return pulse amplitude is related to the attenuation both in the line as well as at the anomaly itself.
There is a trade-off between the pulse width and resolution in pulse return time, which corresponds to distance or reach along the line or cable. For short cables, a narrow pulse is suitable as it gives high time resolution. When the cable is long, its effective bandwidth is significantly reduced. A narrow pulse into a long cable experiences so much attenuation that the reflections become practically undetectable. For this reason, longer cables require a wider pulse, which has energy at lower frequencies.
Wider pulses are however limited in their the capability to resolve closely spaced multiple discontinuities such as opens, shorts or bridged-taps (BT). One of the possible reasons is due to complications with backscatter. Backscatter is a phenomena where the received voltage of the return signal gradually rises during the launch pulse and then slowly decays after the launch pulse has finished. For longer lines where the launch pulse is long and the return signal received from a fault is small, the decaying voltage backscatter may mask the return signal received from the fault.
As shown in FIG. 1, a typical TDR 100 comprises a pulse generator 1 which is capable of generating pulses with pulse widths in the nanosecond to millisecond range. For balanced lines as in a telephone system, the pulse generator 1 differentially drives the line under test 8 with a differential output line driver 2 through a pair of impedance matching resistors 3a,b. A differential pulse is injected at a start time into the cable or line under test 8. Any faults in the form of impedance discontinuities will cause a reflected wave that will arrive back as a return voltage signal, where it is received by an input of the differential receiver 5 at some arrival time after the start time. The amplifier 6 amplifies the return voltage signal of the reflected wave and transmits it to an analog input 7a of an analog to digital converter (ADC) 7 which produces digitized samples at its output 7b. Digital timing is controlled with some form of synchronous digital timing or timebase generator 4, which provides an enable signal 10 to an enable input 7c and a sampling clock signal 11 to a clock input 7d of the ADC 7. The digitized samples together with a corresponding timestamp from the timebase are fed into a microprocessor 9, analyzed and presented to a user for interpretation.
In standard Pulse TDR, the pulse generator 1 generates pulses with a pulse width ranging from the low nanosecond to the low microsecond range. Examples of traces obtained with this TDR technique are shown in FIGS. 2a, 2b and 2c for an open, a short and a bridged tap fault, respectively. The return voltage signal [volt] is plotted on the x-axis against distance [feet] along the line under test 8.
Standard Step TDR uses a pulse width which is of relatively longer duration, from about several microseconds up to a millisecond or more. These pulses have a fast rise time in order to provide energy over a broad frequency spectrum, permitting faults to be detected over an extended range of cable lengths. Examples of traces obtained with the Step TDR technique are shown in FIGS. 3a, 3b and 3c for an open, a short and a bridged tap fault, respectively. The return voltage signal [volt] is plotted on the x-axis against distance [feet] along the line under test 8.
The return signals of a Pulse TDR are easier to interpret so by and large they are generally preferred in the telecom test equipment industry. With Step TDR the return signals become considerably more difficult to interpret as the complexity for a network of cables increases.
Standard numerical differentiation of a Step TDR pulse is a well known technique. The pulse generator 1 generates a pulse with a pulse width or duration that is longer than the time required for a pulse to propagate to the end of the line under test 8. The ADC 7 samples the voltage of the return signal at regular intervals with a constant sampling frequency to produce raw data in the form of a series of digital voltage samples. The microprocessor 9 stores each digital voltage sample of the raw data as an element in a data array for further analysis and post processing.
The standard differentiation of the stored raw data is performed numerically in the microprocessor 9. Acquisition times of consecutive digital voltage samples differ by a sampling period. Thus a fixed preset time offset between any two digital voltage samples corresponds to an difference in an array index between the stored elements in a data array. The differentiation proceeds by a pair-wise numerical subtraction of the elements of the array whose indices are offset by an integer multiple of the sampling period equal to the preset time offset. Alternatively the time offset may be deduced from the difference in recorded timestamps between the respective digital voltage sample pairs in the data array.
Exemplary traces obtained with the numerically differentiated Step TDR technique are shown in FIGS. 4a, 4b and 4c for an open, a short and a bridged tap fault, respectively. The return voltage signal [volt] is plotted on the x-axis against distance [feet] along the line under test 8. The resulting traces resemble those of the classical Pulse TDR where the time offset corresponds to the pulse width used in FIGS. 2a, 2b and 2c. 
There are some practical limitations of the above technique that stem from the attenuation, pulse spreading and slower rise times of the return signals when the fault to be detected is located at long distances in the line under test 8.
An integrated TDR for locating transmission line faults is disclosed by Williams in U.S. Pat. No. 6,714,021 issued Mar. 20, 2004. The integrated TDR comprises an integrated circuit with a transmitter, a path coupled to the transmitter, and a TDR receiver integrated with the transmitter for analyzing a reflected signal from the path. The TDR receiver compares the reflected signal with a variable reference signal to generate a logic state at a sampling instant determined by a timebase generated by a sampling circuit. The reflected signal equals the variable reference signal when the logic state transitions. The reference signal and the corresponding timebase value are recorded at the logic state transition. A waveform is generated from the recorded reference signal and its corresponding timebase value. A reference point for the waveform is determined. The location of a fault on the transmission line can be determined from the timebase value difference between the reference point and the fault.
However, the number of timebase and gain ranges required to display the acquired traces increases with the length of the cable to be inspected. This can be not only labor intensive and time consuming, but also carry an increased risk of overlooking or missing the detection of some faults.
It is an object of the invention to provide an enhanced TDR method for improving the reliability of detection of faults particularly in long lines or cables.
Another object of the invention is to improve the user interface of acquired data to reduce the number of gain and distance ranges that a user needs to select on a TDR display or other output device in order to inspect a line.
Of particular importance for hand-held instruments, a further object is to provide a specified distance resolution within the constraints of electrical power consumption and consequently limited speed of operation for the TDR electronics, such as a microprocessor clock rate or an analog to digital converter sampling rate.